This invention relates to a digital filter used in applications such as signal processing and control systems and, more particularly, to a recursive filter whose transfer function adapts to the operating environment of the filter.
Electronic filters have a variety of uses, including: echo cancellation in telecommunications, control systems, and signal noise reduction. The basic electronic filter allows a preselected frequency bandwidth to be transmitted by the filter, while attenuating all other bandwidths.
Heretofore various digital and recursive filters have been developed. See, for example, U.S. Pat. Nos. 4,305,133; 4,811,263; 4,920,507; 4,920,530; 4,928,258; 5,014,232; and 5,089,981.
There are two broad categories of electronic filters, Finite Impulse Response (FIR) filters and Infinite Impulse Response (IIR) filters. FIR filters are built around a series of unit delay elements connected so that the first delay element delays the input signal by one time unit, the second delay element delays the input signal by another time unit, and so on, until the input signal coming out of the final delay element is delayed by the number of time units equal to the number of delay elements in the series. A copy of the output of each delay element is tapped out and multiplied by a predetermined weighting factor. The weighted outputs from all of the delay elements are serially added using summation elements. The output of the last summation element in the series is the output of the filter.
The FIR filter is characterized by an output that stays within finite bounds when responding to a unit impulse function input. FIR filters are inherently stable, but as one grows in complexity its response time slows proportionately.
The IIR filter also utilizes a delay element. However, it feeds back a copy of the output from the delay element, multiplied by a predetermined feedback parameter, to the input of the delay element. This creates a recursive filter. The IIR filter has a rapid response time, but is not inherently stable.
Adaptive filters are filters with circuitry that determines the difference between desired output and actual output and then influences the circuitry of the filter to correct for the difference between the actual output and the desired output of the filter. An adaptive FIR filter multiplies the tapped output of the delay elements by adaptive weighting factors. This type of filter is relatively stable and exhibits a relatively small error. However, it is limited by many of the same limitations of the conventional FIR filter.
An improvement on the adaptive FIR filter is a partially adaptive IIR filter. One example of this type of filter is disclosed in U.S. Pat. No. 5,014,232. However, because the feedback of the filter is not modified by any adaptive parameters, the output of the filter is not fully optimal and the signal output error is not minimized. The filter disclosed therein uses a series of IIR filter cells, the outputs of which are each multiplied by an adaptive weighting factor, which brings the actual output of the filter more closely in line with the desired output.